The presence of oxytocin and mesotocin in the hypothalamus of two Australian marsupials, the bandicoot (Isoodon macrourus) and the brushtail possum (Trichosurus vulpecula), was examined by immunocytochemistry. Tissue was fixed in paraformaldehyde in phosphate buffer and immunoreactive cells were detected using highly specific rabbit antioxytocin and sheep anti-mesotocin as primary antisera. Immunoreactive oxytocin cells were demonstrated in the paraventricular and supraoptic nuclei of bandicoot and possum hypothalami, with greater density being observed in paraventricular nuclei. Immunoreactive mesotocin cells were also found in both hypothalamic nuclei of the possum but not of the bandicoot. The same cells appeared to stain for both peptides.